Civ on high difficulty is essentially something I see as an impossibility, and don't even consider.

Beyond that... hmm. Bayonetta, because I suck with QTE's, perhaps. Actually it wasn't hard, more just frustrating. Hard games aren't very common these days. I suppose I could point to games like Pacman and Tetris which are impossible to win but some people can do so much better at them than I can. And playing Hotline Miami was addictive despite how tricky it could be. It's definitely doable, if only because each level is actually really, really short, but the amount of times I had to attempt some of the levels was excessive. xD

Hardest learning curve? X3. I just couldn't figure out what I was to do and/or how I was supposed to do it. With Dwarf Fortress at least I had fun deciphering it. With X3 it was just frustrating.